you know you generally have to change voltage when you overclock a card right? Usually not until you get a bit higher in the mhz but maybe thats your problem.
XFX 550W should be more than enough, but I'd like to know which PSU you have now, you know, for not tricking you to buy a new PSU when the problem isn't even that. I just find it weird that that card can't even push 50MHz, that'd be some serious silicon lottery misfortune!